The present invention relates to a nozzle flapper mechanism, and more particularly to a nozzle flapper mechanism for use in an electropneumatic transducer for converting an electric signal to a fluid pressure signal, especially a pneumatic pressure signal, the nozzle flapper mechanism having, as a transducer element, a nozzle flapper being in the form of a bimorph electrostrictive (piezoelectric) device, and being capable of converting a low-voltage electric signal accurately to a pneumatic pressure signal.
Heretofore, torque motors have widely been used as a device for converting an electric signal to a pneumatic pressure signal. An electric current is supplied to the coil of the torque motor to produce a corresponding rotational movement which is converted to an amount of displacement for conversion into a pneumatic pressure signal through a nozzle flapper, a pilot valve, or the like. Where a control device such as an electropneumatic transducer is constructed of such a torque motor, the greater the torque produced by the torque motor, the higher the resistance the control device has against mechanical vibrations or disturbances and the more stable the control device is.
In view of the recent trend toward smaller and lighter control devices, it is very important to construct the torque motor in a small size. However, the smaller the torque motor, the lower the torque that can be generated by the torque motor dependent on the value of the electric current supplied thereto. As a result, the control device would be susceptible to mechanical vibrations or other disturbances, and it would be technically impossible to employ any torque motor in certain applications in which the control device is supposed to be used.
The inventor filed U.S. patent application Ser. No. 729,188 for an electropneumatic transducer unit which employs an electrostrictive device for converting an electric signal to a pneumatic pressure signal.
According to the invention of the above U.S. patent application, the electrostrictive device is of the bimorph type in the form of a thin rectangular shape having one end fixed and the other as a free end. When a voltage is applied to electrodes with the electrostrictive device fixed at one end, the free end thereof is slightly displaced. Therefore, where the electrostrictive device is constructed as a nozzle flapper, it can convert a variation in the applied voltage easily to a nozzle back pressure, i.e., a pneumatic pressure signal.
The electrostrictive device as a nozzle flapper may be fixed in position as follows: As shown in FIGS. 1 through 3 of the accompanying drawings, a nozzle flapper mechanism 2 includes a base 4 on which a first column-shaped fixing plate 6a is disposed. An electrostrictive device 8 is placed on the first fixing plate 6a. One end of the electrostrictive device 8 is fastened to the first fixing plate 6a by a second column-shaped fixing plate 6b which is coupled to the first fixing plate 6b by means of a pair of fastening screws 10 threaded into threaded holes in the first fixing plate 6a. A nozzle 12 is fixedly mounted on the base 4 and has a distal upper end positioned closely to the electrostrictive device 8. The other end of the electrostrictive device 8 remote from the first and second fixing plates 6a, 6b is not limited in motion, but serves as a free end.
The first and second fixing plates 6a, 6b may be made of an electrically insulative material or an electrically conductive material. Where they are made of an electrically conductive material, it is necessary to place insulating sheets on them in contact with electrodes of the electrostrictive device 8. An auxiliary plate 14 made of ceramics or the like is attached to the free end of the electrostrictive device 8 for preventing the electrostrictive device 8 from being damaged by the nozzle 12 when the electrostrictive device 8 is flexed when a voltage is applied thereto.
The electrostrictive device 8 is of the construction shown in FIG. 1. A rectangular shim 16 made of an electrically conductive material such as phosphor bronze stainless steel, or the like. Piezoelectric ceramics members 18a, 18b are bonded to upper and lower surfaces, respectively, of the shim 16 by an adhesive. Thin-film electrodes 19a, 19b are placed on upper and lower surfaces, respectively, of the piezoelectric ceramics members 18a, 18b. The piezoelectric ceramics members 18a, 18b cover most of the surface areas of the shim 16. When the electrostrictive device 8 is fixedly positioned by the fastening screws 10, the electrostrictive device 8 which is composed of the electrode 19a, the piezoelectric ceramics member 18a, the shim 16, the piezoelectric ceramics member 18b, and the electrode 19b, arranged in the order named in the upward direction, is firmly gripped between the first and second fixing plates 6a, 6b.
In operation, a DC voltage is applied to the electrostrictive device 8. Since the piezoelectric ceramics member 18b tends to extend and the piezoelectric ceramics member 18a tends to shrink, the free end of the electrostrictive device 8 is flexed largely toward the nozzle 12 and approaches the orifice of the nozzle 12. Since the amount of flexing displacement of the electrostrictive device 8 is proportional to the applied voltage, the nozzle flapper mechanism can produce a fluid pressure signal dependent on the applied voltage signal.
Generally, it is known that when a voltage is impressed on the electrostrictive device 8, the applied voltage and the amount of displacement of the free end of the electrostrictive device 8 are related to each other as shown in FIG. 4. As the applied voltage is gradually increased from 0, the electrostrictive device 8 is deformed along a curve from a to b in FIG. 4. When the applied voltage is thereafter lowered, the displacement curve goes from b to c, but not from b to a. When the voltage is applied again, the electrostrictive device 8 is deformed along another curve from c to b.
Stated otherwise, when the applied voltage is lowered and then increased again, the deformation of the electrostrictive device 8 undergoes hysteresis. This phenomenon of hysteresis particularly manifests itself with the bimorph electrostrictive device. Even if the same voltage is applied, the displacement of the free end of the electrostrictive device 8 is varied with time as indicated by a shift from the curve from b to c to a curve from b' to c'.
When a constant voltage remains applied to the electrostrictive device 8 for a certain period of time, the displacement of the free end of the electrostrictive device 8 is increased with time as represented by the solid-line curve in FIG. 5, the phenomenon being referred to as creeping of the electrostrictive device. The creeping is understood as depending upon the material of the piezoelectric ceramics, the thickness and material of the shim, the adhesive used, the manner in which the electrostrictive device is fixed, and other parameters. The inventor has made various attempts to avoid the creeping, and found that a displacement caused by the creeping ranges from 10% to several tens %. The creeping is liable to be promoted when the ambient temperature varies.
The creeping is caused largely by the characteristics which the electrostrictive device itself has. On the other hand, it has been confirmed that the creeping is also developed by the fact that the piezoelectric ceramics members 18a, 18b are fixed to the shim 16 by the adhesive, and that the first and second fixing plates 6a, 6b are curved at their opposite ends by the strongly tightened fastening screws 10 which are threaded into the base 4 through relatively large holes defined in the fixing plates 6a, 6b, thus pressing the corners of the electrostrictive device 8 as shown in FIG. 6. When a relatively low voltage, say about 10 V, is applied to the electrostrictive device 8 for fine controlling action, any displacement caused on the electrostrictive device 8 is very small, resulting in the need for controlling movements for a distance on the order of a few microns. Accordingly, the forces with which the fastening screws 10 are to be tightened have to be adjusted very carefully.
However, it is virtually impossible to tighten the two fastening screws 10 with equal forces, and the electrostrictive device 8 is usually subjected to irregular stresses due to uneven screw tightening forces applied.
Furthermore, the portions of the piezoelectric ceramics members which are gripped by the fixing plates are also expanded and contracted by the application of a voltage thereto. Internal stresses developed in the piezoelectric ceramics members are therefore varied, causing the creeping which will make it difficult to effect accurate control. More specifically, upon application of a DC voltage to the electrostrictive device, the portions of the piezoelectric ceramics members which are held against the electrodes are expanded and contracted. With the piezoelectric ceramics members being tightened by the fixing plates, the above expansion and contraction cause stresses to be varied slightly in the tightened portions of the piezoelectric ceramics members. It has been confirmed that the electrostrictive device is expanded and contracted in a direction normal to the direction of forces With which it is tightened by the fixing plates, thereby producing local "slippage" of the electrostrictive device with respect to the fixing plates. Such slippage is also greatly responsible for the creeping.
The use of an electrostrictive device for the conversion of an electric signal to a pneumatic pressure signal is to achieve a desired signal conversion capability through accurate and fine electric control. Where the creeping as described above exists, however, even if a control voltage applied to the electrostrictive device is kept at a constant level to develop a fixed nozzle back pressure, the nozzle back pressure tends to be varied by the creeping. It would be essentially difficult to effect accurate control of a valve body or the like using a nozzle back pressure if the displacement of an electrostrictive device based on the creeping reached several tens %.